1. Field of the Invention
The present invention relates to a beam type safety device to be used for a press and the like machines, and in particular, a beam type safety device with improved light interception detection speed when a light beam is intercepted.
2. Description of Prior Art
Beam type safety devices have been used in various industrial machines, such as a press, to detect the entry of the operator's hand or the like into a specified hazardous area of the machine.
FIG. 1 shows a typical circuit configuration of the prior art beam type safety device, in which a plurality of light emitting diode LED1 to LEDn are for providing a beam screen formed by a plurality of light beams in a specified hazardous area (not shown), and a plurality of phototransistors PHT1 to PHTn are for receiving lights emitted from the light emitting diodes LED1 to LEDn respectively. That is, the plurality of light emitting diodes LED1 to LEDn and the plurality of the corresponding phototransistors PHT1 to PHTn are disposed so as to face to each other across the specified hazardous area.
Outputs of the phototransistors PHT1 to PHTn are fed to amplifiers A1 to An respectively. The amplifiers A1 to An output a low level signal when the corresponding phototransistors PHT1 to PHTn are in the light receiving state. The amplifiers A1 to An output a high level signal when the corresponding phototransistors PHT1 to PHTn are in the light non-receiving state, namely, when the corresponding light emitting diode LEDk does not emit light or when the light beam emitted from the light emitting diode LEDk to the phototransistor PHTk is intercepted. The light emitting operation by the light emitting diodes LED1 to LEDn and the light receiving operation by the phototransistors PHT1 to PHTn are sequentially scanned by a scanning circuit 10 comprised of a clock pulse oscillator 11, a counter 12, a decoder 13, and a driver 14. The counter 12 counts pulse signals of a specified frequency generated by the clock pulse oscillator 11, the count value of the counter 12 is decoded by the decoder 13, and the output of the decoder 13 is fed to the light emitting diodes LED1 to LEDn via the driver 14. Thus, the light emitting diodes LED1 to LEDn are sequentially lit by the drive pulse outputted from the driver 14.
The drive pulse outputted from the driver 14 is fed to the amplifiers A1 to An. As a result, the amplifiers A1 to An perform amplification of the outputs of the phototransistors PHT1 to PHTn synchronized with the lighting of the light emitting diodes LED1 to LEDn. Thus, the light receiving operation of the phototransistors PHT1 to PHTn are performed. The output of the amplifiers A1 to An are fed to respective hold circuits H1 to Hn comprised of a resistor R and a capacitor C. The hold circuit H1 to Hn, when the output of the amplifier A1 to An becomes low level as a result of light reception at the phototransistor PHT1 to PHTn, is kept at low level until the next scan.
Referring to FIG. 2, when light receiving signals as shown in (a) to (c) are output from the amplifiers A1 to An corresponding to the scan of the light emitting diodes LED1 to LEDn, these signals are transformed into waveforms shown in (d) to (f) due to the time constant by the resistor R and the capacitor C of the hold circuits H1 to Hn, and the outputs of the hold circuits H1 to Hn are held at the low level. Dash lines of (d) to (f) of FIG. 2 indicate the threshold levels for determining if the level is the low level. The outputs of the hold circuits H1 to Hn are integrated through diodes D1 to Dn respectively, and further fed to the base of a transistor Tr1. Here, when all light beams from the light emitting diodes LED1 to LEDn to the phototransistors PHT1 to PHTn are not intercepted, a voltage generating at a bias setting resistor R2 is at the low level, and the transistor Tr1 is in non-conductive state. The collector of the transistor Tr1 is fed to the base of a transistor Tr2 through a resistor R4 and a diode D. The capacitor C is connected between the junction point of the resistor R4 and the diode D and the ground. When the transistor Tr1 is non-conductive, a voltage V1 is at the collector of the transistor Tr1, the transistor Tr2 becomes conductive via the voltage V1, and a light interception detecting relay coil L connected to the collector of the transistor Tr2 is excited. That is, the detection condition is such that none of the light beams from the light emitting diodes LED1 to LEDn to the phototransistors PHT1 to PHTn is intercepted.
On the other hand, when any of the light beams from the light emitting diodes LED1 to LEDn to the phototransistors PHT1 to PHTn is intercepted, the output of the amplifier Ak for amplifying the output of the phototransistor PHTk relevant to the intercepted beam stays at the high level, the output of the corresponding hold circuit Hk becomes the high level accordingly, the transistor Tr1 becomes conductive thereby, the collector of the transistor Tr1 the low level due to the presence of a collector resistor R3 of the transistor Tr1, the charged electricity is discharged, the transistor Tr2 becomes non-conductive, and the light interception detecting relay coil L becomes non-excited. Thus, the light interception detection state occurs.
In the case of prior art beam type safety device comprised of aforementioned circuits, as apparent from FIG. 2, the time constant should be set longer than a single scan time for the light emitting diodes LED1 to LEDn, and as a result, it takes a considerably long time from the occurrence of light interception for the light interception detecting relay coil L to become non-excited state. This tendency becomes more remarkable as the number of light beams (optical axes) increases. This fact is a problem not neglectable for a safety device whose purpose is to promptly detect the entry of the hand or the like to the hazardous area, and involves the possibility of resulting in a serious accident.
In addition, in the case of the prior art circuit, to supress the noise, an intergrating circuit comprised of the resistor 4 and the capacitor C2 must be inserted, further lengthening the light interception detection time.